I completely understandable why you ask, but this question is the wrong way around. A more useful question is "will plugins that work for 3.5 work on version 5?".
This is because there is no way Confluence developers can be expected to know what plugin developers may have done and code to support them, it's a bottomless bucket of worms which they can't code for. Confluence does not "support" any plugins, it's the other way around - the plugins need to "work with confluence"
The short answer to that more useful question is "no". Some might work as they are, but most will need to be recompiled against the updated API. For a large jump such as this, many will need code adjustments before they work.
You will be relying on the authors of each individual plugin to update their plugin to match the 5.x APIs. Not all of them will (especially if a plugin is made redundent by new features)
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